1. Field of the Invention
The present invention relates to a sewing machine including a sheet holding device for holding one or more work sheets and a moving device for moving the sheet holding device, and thereby moving the work sheet or sheets, relative to a sewing needle.
2. Related Art Statement
U.S. Pat. No. 4,455,956 discloses a sewing machine including (a) a pair of electric motors which are provided under a sewing bed such that respective drive shafts of the motors extend perpendicular to a top surface of the bed, (b) a pair of sector spur gears which are engaged with respective pinion gears fixed to the drive shafts of the motors, (c) a first group of levers which cooperate with each other to move a feed plate (i.e., sheet holding device) in a first direction (i.e., X direction) parallel to the top surface of the bed, according to the rotation of one of the spur gears, and (d) a second group of levers which cooperate with each other to move the feed plate in a second direction (i.e., Y direction) parallel to the top surface of the bed, according to the rotation of the other spur gear.
In the above sewing machine, the feed plate is movable to any desired position on an X-Y plane defined by the X and Y directions, when one or both of the two motors is/are driven by an appropriate angular amount or amounts.
However, the transmitting mechanism for transmitting the drive forces of the motors to the sheet holding device (i.e., feed plate), including the spur gears and the first and second groups of levers, suffers from small reduction gear ratio. Therefore, the moving device for moving the sheet holding device needs a motor or motors which can output a high power or torque. However, the high-power motors have a large size and cost high.
On the other hand, if a drive-force transmitting mechanism including a plurality of steps each for increasing the reduction gear ratio is employed as part of the moving device, then the moving device suffers from increased inertia thereof. When the moving device is started to move the sheet holding device, is stopped, or is direction-changed, the inertias of the sheet holding device and the moving device react on the motors, thereby adversely changing the rotation amounts of the motors. For avoiding this, the moving device needs expensive high-power motors that, however, occupy a large space under the sewing bed where it is difficult to provide a wide space. The reacting motion of the sheet holding device transmitted backward to the motors is directly proportional to the square of the moving speed of the sheet holding device or feed plate. Thus, in the prior sewing machine, the moving device could not move the feed plate at high speed and accordingly could not effectively follow the high rotation speed of the needle-driving motor.
Japanese Patent Application laid open for opposition under Publication No. 60-17548 discloses a sewing machine including a belt and a pulley for moving a feed plate. In this sewing machine, too, the motion of the feed plate is adversely transmitted to the output shaft of a drive motor via the belt and the pulley. Thus, the second sewing machine suffer from the same problems as described above with the first sewing machine.